The invention relates to a device for locking and releasing the mandrel-holding carriage of a piercing pressure mill. The device comprises at least one lever having an end pivoted to a horizontal pivot perpendicular to the mandrel axis, the other end of the lever being free and releasably engaging an abutment formed in the carriage.
A piercing pressure mill, hereinafter denoted by the symbol PPM, is a machine for converting a square bar into a corresponding round, axially perforated bloom. A PPM mainly comprises a pair of rolling units, including rollers having round grooves, a mandrel having a drill bit extending along the rolling axis from the outlet side of the pair of rolling units, and a thrust member constructed and dimensioned so as to drive a square bar between the grooves of the pair of rolling units against the drill bit.
The mandrel or chuck is borne by a carriage movable along the rolling and drilling axis in guides formed on a rigid structure forming part of the PPM bearing structure and disposed at the outlet side thereof. In order to drill the bar axially as required, the carriage is rigidly held in the operating position by locking means or devices which firmly resist the action of the thrust means.
It is also known that a PPM can be adapted to convert a square bar into a round bloom which is not completely drilled but has a closed end. In a widely-used system for obtaining a bloom of the last-mentioned kind, the action of the thrust means is stopped when its head is at a predetermined distance from the drill bit.
The aforementioned system has a technical drawback which has not hitherto been satisfactorily overcome, i.e. when the thrust means stops, the bar for rolling stops momentarily in that the rollers do not have sufficient strength to continue drilling against the stationary bit. The momentary stoppage of the bar causes the rollers to become loose, even though only for an instant, from the bloom, thus varying the torque in the roll-actuating means. This produces a recoil having an intensity depending on the resilience and the clearances in the actuating means, resulting in considerable, known disadvantages and damage, i.e. wear on the actuating means. These disadvantages could be obviated if the mandrel-bearing carriage could be instantaneously released when the thrust means is stopped. However, existing carriage locking means cannot be released instantaneously and are difficult to actuate due to the enormous forces acting drill bit and carriage during the drilling phase.